In mass spectrometry, a solid, liquid, or gas sample contains atoms or molecules that are targets for study, usually quantification or identification. The targeted atoms or molecules are ionized and introduced into a mass spectrometer in the gas phase. The ionized atoms or molecules (ions) are separated according to their charge-to-mass ratio and are detected by a mechanism capable of detecting charged particles. The resulting signals are processed and organized into a spectrum that presents the relative abundance of the different ions as a function of ion mass-to-charge. This information is used for identification and quantification. Identification is accomplished by correlating the detected mass-to-charge to known or expected mass-to-charge. Alternatively, a characteristic fragmentation pattern may be used where ions that result from structural disintegration of the primary molecular structure are similarly separated and detected.
Separation of ions based on the mass-to-charge ratio can be accomplished by many techniques. One such the technique is time-of-flight (TOF) mass spectrometry. In the time-of-flight technique, ions of different mass-to-charge ratios are subjected to constant energy acceleration. The ions are then detected at a distance away from the location of acceleration. At the detection location, the ions will impinge upon a detector at different times that are related to the ion mass-to-charge according to the formula:
  t  =            m              1        /        2              ·    d    ·                  KE        2            
Where:
t is the time required for the ion to travel the distance from the point of acceleration to the detector,
m is the ion mass-to-charge,
d is the distance between the point of acceleration and the detector, and
KE is the energy the ions receive in the acceleration.
With the distance and the energy being constant, the ion flight time will depend on the square root of the mass-to-charge. It is often the case that ions enter the accelerator, and are subsequently accelerated, making the time-of-flight technique a pulsed technique. This means that the ions are created in pulses, such as in the case of laser ionization, or when the accelerating electric field is pulsed (switched on rapidly).
The accelerator is an important component of the time of flight technique. It is usually the case that multiple ions are present during any single acceleration event. The different ions may not share the same location in the accelerator. Thus, a task of the accelerator is to create an accelerating electric field that is the same regardless of the ion location. In other words, the accelerator should have a substantially homogeneous electric field for the active volume of the accelerator, the active volume being the space in the accelerator through which any ion that will be subsequently detected will travel.
A homogeneous electric field is created ideally by two perfectly parallel plates separated by some distance—Z, that have infinite dimensions in X and Y, with a potential difference between them. In practice, dimensions X and Y are finite, and this introduces problems with field penetration from the edges of the plates. Also, one plate is typically replaced by a grid that will allow most ions to pass through. If the plates are spaced with a very small Z spacing, the field penetration will be lessened. However, if for some reason, it is desired to have a large Z spacing, the field penetration will destroy the homogeneity and the accelerator will not apply the same kinetic energy to ions at different locations in the accelerator. In this case, the variation in ion flight times will be large, and the resolving power of the spectrometer will decrease. A strategy to minimize the field penetration from the sides is to use “field homogenizing” plates placed between the original two plates. These field homogenizing plates will have an applied potential that is linearly varying depending on the position between the two original plates. This assembly can be called a ring-stack accelerator (RSA). In the case where the ions are created in pulses, such as in laser ionization, the potential can be applied to the field homogenization plates by a resistive voltage divider network. In this mode, Ohms law will apply. But if the atoms and molecules are ionized elsewhere and introduced into the RSA, the field in the RSA will have to be switched off to allow the ions to enter, then switched on to provide the acceleration. In this situation, the switching on and off will happen very rapidly, and Ohms law will not apply. The voltage division will depend mostly on the capacitance values between all the plates and between the plates and the surrounding environment. It would be desirable to achieve a homogeneous electric field for the situation where the electric field switches on and off in an RSA.